Thermoplastic resins, particularly acrylonitrile-styrene resins (AS resins), and acrylonitrile-butadiene-styrene resins (ABS resins) exhibit excellent physical properties and are therefore widely employed for various types of moldings. However, these types of thermoplastic resins suffer from a poor moldability, release performance, lubricity, and water repellency. On the other hand, polyorganosiloxanes known as "silicone oil" and "silicone gums" exhibit excellent interfacial properties and for this reason are used as additives for improving the moldability, release performance, lubricity, and water repellency of thermoplastic resins. There have already been attempts at blending polyorganosiloxanes into these types of thermoplastic resins for the purpose of improving the properties of the latter (refer to Japanese Patent Publication Number Sho 57-10144. However, polyorganosiloxanes are in general poorly compatible with thermoplastic resins and particularly with AS and ABS resins, and this makes it quite difficult to obtain a homogeneous dispersion of the polyorganosiloxane in a microfine form in such thermoplastic resins. As a result, blends of polyorganosiloxane and thermoplastic resin exhibit a poor processability and yield molded articles beset by a number of problems. Thus, the polyorganosiloxane may bleed onto the surface of the molding; the molding may suffer from a reduced mechanical strength and an impaired appearance due to the phase separation of these two components; and a substantial variation from molding to molding may occur in the moldability, release performance, lubricity, water repellency, and mechanical strength.